This invention relates to electrolytic reduction cells for the production of molten metals from molten salts, where the molten metal density is less than that of the electrolyte, and to methods of operating such cells. More particularly, the invention relates to electrolytic reduction cells of this type having reservoirs for the collection of the molten metal produced by the cells.
Magnesium and, to a lesser extent, lithium metals are normally produced on a commercial scale by the electrolysis of their chloride salts contained in a heated molten electrolyte in an electrolytic reduction cell. As electrolysis proceeds, metal is produced in molten form (since its melting point is lower than the temperature of the molten electrolyte) and, being less dense than the electrolyte, the molten metal floats to the surface of the electrolyte, where it collects and is periodically removed.
Most such reduction cells contain a metal recovery section separate from an electrolysis section. The metal recovery section takes the form of a relatively quiescent section of the cell in which metal separation may proceed effectively. In most cases, a barrier or partition is provided between the electrolysis section and metal recovery section so that the separated metal in the metal recovery section, which floats on the surface of the electrolyte, is maintained out of contact with chlorine gas, the other product of electrolysis. Electrolyte is recirculated from the metal recovery section back to the electrolysis section so that there is always sufficient electrolyte for the electrolysis process. Any barrier or partition provided for this purpose generally has channels or openings below the level of the metal layer to permit such recirculation. To assist in the electrolyte circulation, some electrolytic reduction cells of this type use level control devices to control the liquid level (metal plus electrolyte) in the metal recovery section. For example, an open bottomed bell or xe2x80x9csubmarinexe2x80x9d immersed in the electrolyte which is connected to an inert gas supply may be used, where gas pressure is used to adjust the amount of liquid stored in the bell, which thereby alters the liquid level in the cell.
Modern electrolysis cells, particularly those of the multipolar type, have high productivity, but at the same time generate excess heat which must be removed to maintain the electrolyte temperature at a constant target level. This is often accomplished using an air to liquid heat exchanger, immersed, for example, in the metal recovery section.
The productivity of such multipolar cells has been increased to the point that either the capacity of the metal recovery section must be increased to allow for the storage of more metal between periodic metal removal operations (metal tapping), or alternatively, the frequency of metal removal must be increased. Neither of these solutions is particularly satisfactory. The provision of larger metal recovery sections would mean that cell size would be increased, thus increasing the size of metal production facilities. More frequent metal tapping results in reduced efficiency of cell operation. The very desirable gains in efficiency of metal production are therefore producing their own problems regarding plant design and operation.
Furthermore, modern electrolytic cells for the production of magnesium operate at temperatures very close to the melting point of the electrolyte in order to maximize current efficiency. This means also that the cell operating temperature lies close to the freezing point of the magnesium product. When the magnesium is collected on the electrolyte surface as in conventional cells, it can become semi-solid, or at least very viscous and difficult to tap. The conventional solution to this problem is, by some means, to heat the entire metal pad in the metal recovery section prior to tapping. This, of course, raises the electrolyte temperature and reduces current efficiency for a part of the cell operation. Heat exchangers as described above can be used to maintain the temperature at a relatively constant level, even when extra heat input is used during tapping, but in large capacity cells, the heat exchanger sizes necessary to accomplish this during and after a tapping operation become prohibitively large and expensive and require large cell sizes to accommodate them.
PCT patent publication WO 97/28295, published on Aug. 7, 1997 in the name of Olivo Sivilotti, discloses a process and apparatus for electrolysing metal chloride salts. In this patent document, metal from a metal collection section is circulated to a reservoir provided within the cell submerged beneath the molten electrolyte, and is then periodically tapped from the reservoir. The reservoir is positioned approximately centrally of the cell to ensure proper electrolyte circulation, which is associated with the particular intended method of operation of this particular cell. The central submerged reservoir is provided in order to maintain the molten metal out of contact with the refractory cell walls as much as possible, to prevent reaction with the refractory material and consequent contamination of the metal. The disadvantage of this design is that it is very specialized, complex and consequently expensive. Existing cells cannot easily be modified to accommodate this design. The central location of the reservoir tends to maximize heat equalization between the reservoir and the cell, which can result in reductions of current efficiency.
There therefore is a need for a less complex and more practical solution to the problem of increasing metal storage in metal production cells.
An object of the present invention is to improve the efficiency and ease of production of metal in electrolytic reduction cells where the density of the molten metal produced is less than that of the electrolyte.
Another object of the present invention is to provide a process and electrolytic apparatus for producing molten metals less dense than the electrolyte in which increased volumes of molten metal can be accommodated within electrolysis cells, particularly those with large production capacity, without having to resort to cells of much larger size, to tapping operations at much greater than normal frequency, or to the use of excessively large and expensive heat exchangers
Yet another object of the present invention is to enable molten metal in electrolytic reduction cells to be kept at least temporarily at temperatures above those of the molten electrolyte without reducing cell efficiencies.
According to one aspect of the invention there is provided an electrolysis cell for producing a molten metal having a density less than a density of a molten electrolyte used for producing said metal in said cell, comprising: at least one electrolysis section for the electrolysis of a salt of said metal contained in a molten electrolyte to form droplets of said metal in molten form contained in said electrolyte; electrodes within said at least one electrolysis section for effecting said electrolysis; a metal recovery section for separation of said metal from said electrolyte to form a molten metal layer, having an upper surface, floating on an upper surface of said molten electrolyte; a liquid-filled reservoir communicating with an upper part of the metal recovery section for the collection of molten metal from said molten metal layer by overflow of said layer into said reservoir; liquid transfer apparatus communicating with said reservoir for enabling molten metal from said layer to accumulate in said reservoir by displacement of liquid already present in said reservoir, without removing said liquid permanently from said cell: and a tapping device for periodically removing molten metal from the cell. By xe2x80x9coverflowxe2x80x9d of the molten metal from the recovery section to the reservoir, we do not necessarily mean that the upper surfaces of the molten metal layers in these parts of the cell have different vertical levels. Indeed, these surfaces may be continuous (i.e. at the same vertical level). When this is the case, metal will nevertheless xe2x80x9coverflowxe2x80x9d from the recovery section to the reservoir as a result of the difference of metal layer thickness in the two parts of the cell caused by the effect of the operation of the metal transfer apparatus.
According to another aspect of the invention, there is provided a process of producing a metal, which comprises: electrolysing a salt of said metal contained in a molten electrolyte in an electrolysis section of an electrolysis cell to produce a mixture of molten metal and molten electrolyte; conveying the mixture to a metal recovery section of said cell and allowing the metal and electrolyte to separate into layers in the metal recovery section, said metal in molten form having a density that is less than the said molten electrolyte; recirculating molten electrolyte from the metal recovery section to the electrolysis section; and periodically removing molten metal from the cell; wherein the process includes providing a liquid-filled reservoir communicating with an upper part of the metal recovery section for the collection of molten metal from said molten metal layer by overflow of said layer into said reservoir and liquid transfer apparatus communicating with said reservoir, and wherein the said liquid transfer apparatus displaces liquid already present in said reservoir to enable molten metal from said layer to accumulate in said reservoir, without removing said liquid permanently from said cell.
Preferably the reservoir has a top, sides and bottom, at least one opening in said top or sides communicating with the metal recovery section, at least part of said at least one opening lying below said upper surface of said metal layer during at least part of normal cell operations, all of said at least one opening lying above said upper surface of said electrolyte in said metal recovery section for at least part of said normal cell operations, and said sides and bottom being otherwise closed to prevent the free flow of metal or electrolyte between the said metal recovery section and said reservoir.
Preferably the said at least part of said at least one opening lies below the surface of the metal layer in the said metal recovery section during all normal cell operations.
The sides of the reservoir may be formed by several adjoining side walls (e.g. as in a rectangular container) or a single continuous side wall (e.g. as in a cylindrical container).
According to another aspect of the invention, there is provided an electrolysis cell for producing a molten metal having a density less than a density of a molten electrolyte used for producing said metal in said cell, the cell comprising: at least one section for the electrolysis of a salt of said metal contained in a molten electrolyte to form droplets of said metal in molten form contained in said electrolyte; electrodes within said at least one electrolysis section for effecting said electrolysis; a metal recovery section for separation of said metal from said electrolyte to form a molten metal layer, having an upper surface, floating on an upper surface of said molten electrolyte; a reservoir for withdrawal and temporary holding of molten metal separated from said electrolyte in said metal recovery section; and a tapping device for periodically removing molten metal from the cell; wherein said reservoir is in the form of a container having at least one opening communicating with the metal recovery section, at least part of said at least one opening lying below said upper surface of said metal layer during normal cell operations, all of said at least one opening lying above said upper surface of said electrolyte in said metal recovery section for at least part of said normal cell operations, and said container being otherwise closed to prevent the free flow of metal or electrolyte between the said metal recovery section and said reservoir.
The container forming the reservoir preferably has a top, sides and a bottom. Preferably the at least one opening is in the top or sides of the container. The top may be completely open, thus forming the opening between the reservoir and the recovery section.
According to another aspect of the invention, there is provided a process of producing a metal, which comprises: electrolysing a salt of said metal contained in a molten electrolyte in an electrolysis section of an electrolysis cell to produce a mixture of molten metal and molten electrolyte; conveying the mixture to a metal recovery section of said cell and allowing the metal and electrolyte to separate into layers in the metal recovery section, said metal in molten form having a density that is less than the said molten electrolyte; recirculating molten electrolyte from the metal recovery section to the electrolysis section; and periodically removing molten metal from the cell; wherein the process includes providing a molten metal reservoir in the cell in the form of a container having at least one opening communicating with the metal recovery section, and maintaining an upper surface of said metal layer in said metal recovery section above at least part of said at least one opening during normal cell operations, maintaining an upper surface of said electrolyte in said metal recovery section below all of said at least one opening for at least part of normal cell operations, and wherein said electrolyte or said metal cannot otherwise freely flow between said metal recovery section and said reservoir.
Preferably all of the said at least one opening lies above the surface of the electrolyte for at least 80 percent of the time that the cell operates under normal cell operations. More preferably, all of the said at least one opening lies above the surface of the electrolyte during substantially all normal cell operations.
The said at least one opening may be partially above the level of both the said electrolyte and the said metal layer in the said metal recovery section during normal cell operation.
Most preferably the reservoir has a top which is completely open so that the reservoir is in the form of an open topped container having solid side walls and a solid bottom wall, wherein at least a portion of the side walls lies below the surface of the metal layer in the metal recovery section during normal cell operations, but the side walls lie entirely above the upper surface of the electrolyte in the metal recovery section during normal cell operations. A part of the side walls lies above the metal layer as well during normal cell operations, but the side walls may also be completely immersed below the top surface of the metal during normal cell operations.
The liquid transfer apparatus which displaces liquid in the reservoir most preferably does so without removing the liquid from the cell at all (even temporarily). However, temporary removal of the liquid may be desired in some cases for convenience, e.g. the liquid may be routed outside the cell from one point in the cell to another. Hence the displacement of liquid from the reservoir may be routed via a path within the cell or passing temporarily outside the cell. Preferably the liquid transfer apparatus is operable only during the portion of normal cell operations in which part of the said at least one opening lies below the surface of the metal in the metal recovery section.
The liquid transfer apparatus may be, for example, a bell or submarine within the reservoir connected to an external gas supply, where the gas pressure can be adjusted to displace liquid from the reservoir into the bell or submarine. It is particularly preferred that the liquid transfer apparatus draw liquid from the reservoir and return it to the metal recovery section. A pump will generally be used to accomplish this. Such a pump can be of any form compatible with the cell environment. A gas lift pump or impeller driven draft tube may be used. A pump in which liquid is alternately drawn into and expelled from a chamber by means of application of vacuum and gas pressure, and the flow is controlled using check valves may also be used. It is also possible to use centrifugal pumps for such an application. The pump may feed a secondary storage reservoir or surge volume or similar container from which it is flows back into the metal recovery section. Preferably, the liquid in the reservoir is displaced or removed from a point at least half way down the reservoir and most preferably from at or near the bottom.
Normal cell operations refer to cell operating conditions that occur during the major portion of time the cell operates, and excludes start-up and shut-down operations, and short perturbations to metal and electrolyte levels that may be associated with tapping or metal from the cell or adding electrolyte and metal salt.
The tapping device is preferably a syphon for metal removal and is used to remove metal from the reservoir. Pumping devices including centrifugal pumps and pumps operating by cyclical suction and pressure may also be used to remove metal from the cell.
The metal to which the invention is applicable is preferably one of magnesium, lithium, sodium, calcium and mixtures thereof. Most preferable the metal is magnesium.
Preferably the electrolysis section and metal recovery section are separated by means of a partition or barrier which prevents the gaseous products of electrolysis from entering the metal recovery section and which has openings to permit circulation of electrolyte.
The function of the molten metal reservoir is to temporarily store more metal from the cell than can conveniently be held in the metal recovery section as a layer floating on the molten electrolyte. The reservoir may form an integral or internal part of the cell or be separate from it. If separate from the cell, it may be in the form of an insulated container attached to an outer wall of the cell. The insulated container may be a refractory container or a steel container with an insulating material on its surface. However, the reservoir is preferably an integral or internal part of the cell and is positioned immediately adjacent to or within the metal recovery section and separated from it only by a wall or walls (normally a wall or walls of the reservoir itself). The bottom wall of the reservoir is preferably positioned considerably vertically below the normal uppermost position of the molten electrolyte in the metal recovery section so that the reservoir acts as a xe2x80x9cwellxe2x80x9d, i.e. a storage area into which the molten metal can flow to a depth greater than that achievable in the metal recovery section. This allows the metal storage capacity to increase without undue increase in the overall size of the cell. To provide for maximum storage capacity it is advantageous to position the bottom wall of the reservoir at the bottom of the metal recovery section, and in certain embodiments, the bottom of the metal recovery section may form the bottom wall of the reservoir. In fact, the metal reservoir may be divided out from the metal recovery section of an existing cell by building a suitable separating wall across an end or a corner of the former metal recovery section. It may also be in the form of a box within the metal recovery section without any common walls with the metal recovery section. This reduces the volume of electrolyte available for the cell, but not sufficiently that operation is impaired or that normal circulation of electrolyte between the metal recovery section and the electrolysis section is significantly affected. In all cases, it is advantageous to position the reservoir in a portion of the metal recovery section where the electrolyte flow is relatively quiescent. This typically will be at a point distant from the channels or passages communicating with the electrolysis sections. Whilst the reservoir may lie within the metal recovery section or be divided out from it by use of dividing walls, it is operationally distinct from it in that while the metal recovery section is used to separate the small droplets of molten metal from the electrolyte to form a layer on the surface, the reservoir is used to collect some or all of this already formed layer and to retain it prior to tapping. The cell otherwise works in the same way as a conventional cell not having a molten metal reservoir.
By collecting the metal in such a reservoir, the metal does not cool as readily by radiation. Because any additional heat required to heat the metal prior to tapping is applied in a more confined space, the thermal effects on the cell are reduced permitting smaller heat exchangers to be used.
The reservoir can be made from any material compatible with the cell environment. Steel may be used and because during normal cell operations the reservoir operates with a similar hydrostatic head inside and out (any head difference being caused only by the difference in the relative levels of metal to electrolyte inside the reservoir and outside the reservoir) the reservoir geometry can be chosen to optimize space without the necessity to reinforce or specially design the reservoir to overcome possible collapse at high temperatures. Steel is advantageous as well in that a removable reservoir can be thereby constructed to permit the reservoir to be periodically removed from the cell for servicing or replacement.
When the metal reservoir is positioned immediately adjacent to the metal recovery section using a separating wall to divide off part of the metal recovery section for this purpose, it is advantageous if the common wall separating the reservoir and the metal recovery section should form no more than a minor portion (i.e. less than about half) of the exterior vertical walls of the reservoir. This provides minimum area to transfer excess heat to the metal recovery section when the metal in the reservoir is heated immediately prior to periodic tapping.
It may be further advantageous to provide a wall between the reservoir and the metal recovery section having insulating qualities, such as fused cast alumina, alumino-silicate or any material such that the wall is resistant to the molten metal and electrolyte, to further limit the transfer of heat. An insulation factor in the range of 1 to 10 W/mxc2x0 C. is normally suitable for this purpose although this depends on cell design and operating temperatures, etc. This is particularly useful when it is desired to minimize the heat exchanger requirements for temperature control. The refractory wall may be present either as a dividing wall as described above, or as a lining on the inside or outside of a steel container.
Of course, heating of the electrolyte in the metal recovery section by the metal in the reservoir can be avoided altogether by physically separating the reservoir from the metal recovery section. Nevertheless, this is usually not preferred. By positioning the reservoir adjacent to the metal recovery section, heat from the metal recovery section gradually passes through the interconnecting wall (even if it is made of insulating refractory) to keep the metal in the reservoir at a melting temperature without additional heating. The insulating nature and/or limited exposure of the interconnecting wall, however, protects the electrolyte from the occasional and brief increases in temperature of the metal that may be needed prior to tapping.
During operation, as metal is produced in the electrolysis section, it is carried to the metal recovery section where it separates to form a metal layer or xe2x80x9cpadxe2x80x9d on the surface of the electrolyte. During periods of operation where the opening or openings between the reservoir and metal collection section lie below the metal level, metal flows into the reservoir as well. In operation, the reservoir will be filled with metal and electrolyte in differing proportions depending on the tapping cycle. Liquid, which is generally electrolyte, is removed from the bottom portion of the reservoir. This electrolyte may be removed either to a submarine or bell positioned within the reservoir or may be pumped into the metal recovery section. This causes more metal to flow into the reservoir, resulting in a greater depth of metal in the reservoir than in the metal recovery section. When the reservoir contains an amount of metal suitable for tapping, the metal is preferably heated 20 to 50xc2x0 C. above the electrolyte temperature and metal is siphoned from the reservoir. As the metal is siphoned off, liquid is returned to the reservoir, either from the submarine or bell or by overflow from the metal recovery section or from a combination of the two. This liquid may be metal or electrolyte or a combination of the two. Once tapping is completed, the procedure is repeated.
When a submarine or bell is used, the electrolyte in the reservoir may not contact and mix with the electrolyte in the metal recovery section to any substantial extent or at all. This means that the electrolyte composition in the reservoir may differ from the electrolyte composition in the rest of the cell. This can occur through natural changes (e.g. through changes in the magnesium chloride levels), or may be done deliberately (e.g. to provide a different melting point).
The reservoir is designed so that the only portions of the reservoir which freely communicate with the metal recovery section are the openings in the top or sides specifically located with respect to the metal and electrolyte top surfaces described above. Any openings in the sides or bottom which permit free communication between the reservoir and the metal recovery section and which do not meet these requirements will cause the apparatus to fail to collect metal as required and are therefore to be avoided. For example, an opening in the side or bottom which is always below the electrolyte top surface and permitting free communication between the metal recovery section and the reservoir will not permit proper operation of the reservoir. However, certain openings which permit liquid flow in only one direction (for example by use of check valves) and which do not therefore permit metal and electrolyte to freely flow between the reservoir and metal recovery section, may be used without affecting the operation of the apparatus and may be useful in certain types of pumps used to transfer electrolyte from the reservoir to the metal recovery section.
In certain modes of operation, it is possible for the liquid levels in the metal collection section to be low enough that the opening or openings to the reservoir lie above the metal in the metal collection chamber. As metal is produced, the level rises and eventually the metal can flow into the reservoir at which point the liquid removal from the reservoir can be commenced. It is undesirable to operate the means for liquid removal from the reservoir if there is no liquid communication with the metal recovery section because the liquid level imbalance so produced can cause operational difficulties including distortion of the walls of the reservoir. It is preferred, for simplicity and better overall control of operations, to operate the cell with a part of the opening or openings always submerged beneath the surface of the metal in the metal recovery section. This is most conveniently assured by means of a level control device operating in the metal recovery section. This level control device may be a bell, or submarine (similar to the device which may be used in the reservoir for temporary storage of liquid).
During start up of cells of this type, the reservoir will be typically filled with electrolyte. This may be done by adding electrolyte to the reservoir directly as the rest the cell is filled with electrolyte, or by raising the electrolyte level in the metal recovery section temporarily to allow the electrolyte to overflow into the reservoir. If a submarine or bell is used in the reservoir, this filling will preferably be done with the submarine or bell filled with pressurizing gas.
The advantages of the invention are that the capacity of electrolysis cells to store molten metal between tapping operations is increased without substantially increasing the floor space required for such cells and without reducing the current efficiency of the cell during normal operation.